With a gas engine in which a combustible gas such as methane or propane is directly introduced into a cylinder (combustion chamber) to be combusted, the ratio between the fuel gas and air is changed by a gas flow rate adjustment valve disposed at the fuel gas path to the engine. As a result, sure ignition and combustion are allowed. In addition, the output of the engine is changed by adjusting the gas flow rate with the gas flow rate adjustment valve, and a governor actuator for driving the gas flow rate adjustment valve.
Japanese Patent Publication No. 3500047 (Patent Document 1) discloses a technology of adjusting the amount of the fuel gas to be supplied. The accompanying FIG. 8 shows the outline of fuel control and the like of the related-art technology. In FIG. 8, 01 represents an engine; 02 represents a crank shaft of the engine 01; 03 represents a fuel gas pipeline; 04 represents a gas flow rate adjustment valve disposed in the fuel gas pipeline 03; and 020 represents an electronic governor. Whereas, 010 represents a governor controller of the electronic governor 020; and 021 represents a governor actuator for driving the gas flow rate adjustment valve 04.
According to the Patent Document 1, the governor controller 010 compares the detected value of the engine revolution speed inputted from a revolution detector 05 and the set revolution speed, and calculates the fuel gas flow rate equivalent to the revolution speed deviation, and outputs the calculated value to the governor actuator 021. The governor actuator 021 changes the gas flow rate adjustment valve 04 by the amount equivalent to the flow rate deviation. As a result, the gas engine 01 is operated at the set revolution speed.
FIG. 9 is a diagram showing a change in engine revolution speed and a change in governor actuator output after start of the gas engine of the related-art technology. At the revolution speed between the start and the idling revolution speed N2 at the engine 01, there is set a revolution speed transition point N1 at which the output of the governor actuator 021 for controlling the gas flow rate adjustment valve 04, in other words, the increasing rate of the fuel limiting value is changed.
Between the engine stop and the revolution speed transition point N1, the increasing rate α1 of the governor actuator output S from a point a equivalent to the governor actuator output S1 set as the initial limiting value of the fuel gas flow rate at the start to a point b equivalent to the governor actuator output S2 at the revolution speed transition point N1 and the increasing rate α2 of the governor actuator output S from the revolution speed transition point N1 to a point c equivalent to the governor actuator output S3 at the idling revolution speed N2 are set in the relationship of α2>α1.
Then, by setting the fuel gas flow rate limiting value S2 at the revolution speed transition point N1, and the fuel gas flow rate limiting value S3 at the idling revolution speed N2, the amount of the fuel gas to be supplied is prevented from increasing more than necessary during the period until the idling revolution speed N2.
Therefore, the fuel gas flow rate limiting value from the governor actuator can be controlled so as to be changed properly following the increase in engine revolution speed at the start of the engine. This can inhibit the occurrence of the following situation: the fuel gas flow rate sharply increases upon start, so that the gas in the combustion chamber is in a rich state. Accordingly, after starting, a proper amount of the fuel gas is supplied to the engine combustion chamber, resulting in the implementation of the stable start free from the occurrence of misfire or knocking.    Patent Document 1: Japanese Patent Publication No. 3500047.
However, a gas engine using a combustible gas such as methane or propane as a fuel may use, particularly, methane with a low concentration generated in a coal mine or the like as a fuel.
In such a case, there is performed underground ventilation aiming at discharge of methane generated during the underground coal mining operation in a coal mine. The gas discharged to the outside of the underground tunnels is retained to be used as the fuel for the gas engine.
Therefore, the concentration of the retained methane is not invariably constant, and tends to vary.
On the other hand, when the gas is used as the fuel for the gas engine, it is a condition for obtaining an efficient operation that the ratio of air and the fuel gas (CH4) falls within a given range.
For this reason, when the fuel gas discharged from a coal mine or the like is used, the fuel gas is required to be supplied to the combustion chamber of the engine in consideration of the concentration.